The present invention relates generally to wave-shaping circuits, and more specifically to wave-shaping circuits for simple integrated circuit (IC) processes, such as MOS (Metal Oxide Silicon) and MIM (Metal Insulated Metal), that do not have area efficient dielectric capacitors.
According to classical electrical theory the high frequency content of an electrical waveform is related to the time-varying nature of the waveform. In particular, fast changes in waveform amplitude are indicative of increased high frequency content. It is well known that a square wave representation, for example, becomes more accurate as the number of high frequency harmonics included in the representation is increased. An infinite number of harmonics would be required to perfectly represent a square wave. Modern digital devices typically utilize square waves and rectangular waveforms, which have similar characteristics.
Increased high frequency content of an electrical waveform can cause problems in electrical circuits and systems. While higher frequency content of an electrical waveform may do no harm in the circuit utilizing the waveform if rise and fall times are faster than required, it is well known in the art that high frequency signals emit electromagnetic radiation that can couple into adjoining circuits and equipments causing problems such as mis-triggering. For example, a tunable radio receiver in proximity to an operating computer can easily detect the existence of many frequencies utilized in the computer and harmonics of those frequencies. It is thus possible and in fact commonplace for digital devices to interfere with wireless communications.
A number of approaches may be used to reduce the magnitude of this problem. Shielding of higher frequency circuitry is common, wherein metallic barriers to electromagnetic radiation are erected in an attempt to contain the radiation in a restricted volume. Electrical filtering is commonplace, seeking to reduce the high frequency content on signal paths entering or leaving an area containing circuitry or equipment which contain high frequency signals. Shielding and filtering of an area which will be adversely affected by coupled electromagnetic radiation is also commonplace. At a higher level, government agencies often put quantitative restrictions on the level of high frequencies which may be radiated or conducted from a given equipment. All of the above solutions, however, are only successful to varying degrees and add size and cost. It is well known in the art that it is better to not generate high frequencies unless they are required.
If the instantaneous amplitude changes in a perfect square wave are replaced by amplitude changes that occur over a finite time, the higher frequency harmonics can be greatly reduced. The higher frequency harmonic content can be minimized if the fast amplitude changes which occur at the edges of a square wave are replaced by amplitude changes which exhibit a constant time rate of change, and if the remaining corners are rounded off. It is clear that digital circuits require rectangular waveforms which change sufficiently fast so as to obtain the required operation, but it is not normally required that these waveforms be excessively faster than this. It is therefore understood in the art that the rise and fall times generated in digital circuitry may be increased and the corners may be rounded as long as one remains within the bounds of good engineering practice.
The above modification to the rise and fall times of a rectangular waveform is referred to as wave-shaping and is shown in FIG. 1. Wave-shaping may be applied at any physical level of electrical circuitry, such as with discrete components or within integrated circuits. Typically the constant slew rate referred to above is achieved in integrated circuits by charging an on-chip capacitor C1 with a constant current source l as shown in FIG. 2. The voltage is integrated on the capacitor at a constant rate, l=CdV/dT, where l is the fixed current, C is the value of the capacitor, and dV/dT is the slew rate or fixed slope of the capacitor voltage as a function of time. The high frequency content of the shaped waveform is significantly less using wave-shaping.
The remaining wave-shaping step, called edge rounding, is achieved by integrating the constant slew rate onto a second capacitor C2. In a CMOS (Complementary Metal Oxide Silicon) process, a MOS (Metal Oxide Silicon) capacitor can be used because of the relatively high specific capacitance available with good temperature stability and matching.
The wave-shaping process described above is well known in prior art, but it is not appropriate for realizing wave-shaping in simple IC processes that do not have area efficient dielectric capacitors (MOS or MIS). Inexpensive bipolar processes frequently have only junction capacitors which are area efficient but have large temperature and voltage coefficients which result in unacceptable shifting of operational characteristics.
There is thus an unmet need in the art to be able to incorporate stable wave-shaping characteristics into inexpensive bipolar IC processes. Therefore, it would be advantageous in the art to be able to teach a method for wave-shaping which is compatible with the inexpensive bipolar IC processes currently in use.